Magnesium aluminate spinel, MgAl2O4, (hereinafter “spinel”) is a very attractive ceramic material for use in various applications requiring a rugged, tough, scratch resistant, transparent material. Spinel articles have a wide transparency range from visible to 5.5 μm wavelength, and mechanical properties several times greater than that of glass while being remarkably lighter than ballistic glass by a factor of 2 for the same degree of armor ballistic protection.
Some of the applications for which spinel is particularly suited require three-dimensional bodies of complex geometry, such as domes for missiles and the like. Since spinel's hardness makes it very difficult to machine, successfully producing such three-dimensional spinel bodies in a cost-effective manner has proven to be no easy task. Illustrative of prior art attempts to do so are the Maguire et al. U.S. Pat. No. 4,347,210, issued Aug. 31, 1982, and the Roy et al. U.S. Pat. No. 4,930,731, issued Jun. 5, 1990.
Maguire et al. employ a hot forging technique in which a combination of tensile and compressive stresses are used to plastically deform a sintered spinet plate between the two portions of a mold defining a cavity of the desired complex shape to produce a spinel body in the shape of the mold cavity. By its very nature, the Maguire et al. technique requires a lot of material rearrangement resulting in nonuniformity in dome dimensions, especially wall thickness, and also introduces undesirable stresses into the spinel body.
The more conventional approach described by Roy et al. uses spinel powder mixed with small amount of a sintering aid, such as lithium fluoride (LiF), which is formed directly into a dome-shaped sintered spinel body in a die mold first using low pressure cold pressing to effect slight compacting of the powder for ease of handling, followed by densification via hot pressing or pressureless sintering, followed by further densification via hot isostatic pressing. Since Roy et al. do not specify any unique mode of mixing the spinel powder with the LiF sintering aid, it can be assumed that they contemplated nothing more than traditional mechanical dry mixing, such as mortar and pestle, ball milling or attritor milling. However, as has been well documented by Villalobos et al. in U.S. Patent Application Publication No. 2004/0266605, published Dec. 30, 2004, U.S. Pat. No. 7,211,325, issued May 1, 2007, and U.S. Pat. No. 7,528,086, issued May 5, 2009, inhomogeneity and contamination problems associated with mechanical mixing of the sintering aid with the spinel powder prior to sintering have been found to be the leading cause of high product rejection rates in attempting to produce defect-free transparent sintered spinel articles.
A known technique for producing complexly shaped, three-dimensional bodies of sintered ceramic materials other than spinel, is freeze casting. Representative U.S. patents describing this technique are the Herrmann U.S. Pat. No. 3,330,892, issued Jul. 11, 1967 and the Sundback et al. U.S. Pat. No. 5,047,182, issued Sep. 10, 1991. In freeze casting, a ceramic powder is mixed with a sublimable vehicle to form a slurry which is cast and then frozen in a mold of the desired complex shape. The frozen part or compact is then demolded and the vehicle is removed by sublimation, i.e., freeze-drying, to obtain a green body which is thereafter sintered to the final densified product. More recently, both camphene and eutectic mixtures of camphor and naphthalene were described as suitable for use as nonaqueous sublimable vehicles in the freeze casting of alumina articles at or near room temperature, by Araki et al. in J. Am. Ceram. Soc. 87 (10) 1859-1863 (2004) and J. Am. Ceram. Soc. 87 (11) 2014-2019 (2004).
What appears to be lacking from the prior art is any reference to the successful application of the freeze casting technique or any modification thereof to the production of transparent sintered three-dimensional spinel bodies. One of the reasons for this might very well be the need to first develop a more effective ready-to-sinter spinel powder that overcomes the inhomogeneity and contamination problems associated with the mechanical mixing of the sintering aid with the spinel powder prior to sintering. Such a powder and its method of preparation are described and claimed in the commonly owned copending U.S. patent application Ser. No. 12/460,451 of Raouf O. Loutfy et al. entitled “Ready-to-Sinter Spinel Nanomixture and Method for Preparing Same,” filed Jul. 16, 2009, now U.S. Pat. No. 8,313,725, issued Nov. 20, 2012, and which is incorporated herein by reference in its entirety.